The present invention relates generally to pulse compressor systems, and more particularly to pulse compressor systems for generating palindromic polyphase codes.
It is well known in the art of pulse radar systems that in order to obtain a good detection capability against a background of noise, a pulse with a large energy content must be transmitted. Since radar systems are generally peak-power-limited systems, the required energy needed for good detection capability can only be obtained by transmitting a longer pulse. In order to retain radar resolution (range) when transmitting a long pulse with a high average power content, pulse compression techniques are employed. In order to effect such pulse compression, a long pulse typically containing some form of linear-frequency-modulation or stepwise approximation to a linear-frequency-modulation is transmitted. A special pulse compressor or matched filter designed to compress the particular coding on the long pulse is then utilized at the receiver to compress the pulse to permit separation of adjacent range resolution cells.
In many radar applications, it is desirable to change waveform from pulse-to-pulse without changing the characteristics of the matched filtered signal. This is particularly the case in range-doppler-coupled MTI systems.
In general, MTI radar systems are utilized to distinguish between fixed targets and moving targets by means of the doppler effect. Such systems are based on the fact that the radar signal echo reflected from a moving object changes in phase from pulse to pulse due to the radial velocity of the target (the velocity toward or away from the radar receiver). Fixed objects, however, introduce no phase changes from pulse to pulse in the radar echo signal. Thus, fixed and moving targets may be distinguished by comparing the phases of successive echos via a subtraction process. In this subtraction, fixed target echos cancel but moving targets do not cancel due to the phase change from pulse to pulse. Unfortunately, such MTI also cancel echos from targets moving at rates that produce phase changes of integral multiples of 2.pi. radians from pulse to pulse. These velocities are called blind speeds. This phase change process referred to above also produces what is known in the art as range-doppler-coupling in compressed frequency coded waveforms. When range-doppler-coupling is present, the range at which an echo appears will vary the targets doppler in an direction determined by the sign of the doppler shift (the direction of the target motion) and by the direction of the radar's frequency sweep on transmission. This range-doppler-coupling phenomena is also used to develop MTI systems without blind speeds.
Most prior art range-doppler-coupling MTI systems utilize a two pulse technique to detect moving targets. This technique comprises transmitting either simultaneously or back-to-back an upswept chirp or stepped approximation to a chirp and a downswept chirp or stepped approximation thereto. By way of example, see U.S. Pat. No. 3,905,033 to Moore et al.
It can be seen that in order to effect an accurate subtraction of these two pulses so as to cancel nonmoving target echos and to yield only moving targets, it is highly desirable to have upswept coded pulses and downswept coded pulses which have almost identical autocorrelation function characteristics. In the general case, it is highly desirable to provide coded pulses whose autocorrelation functions are identical under frequency sweep reversal but where the cross-correlation between non-frequency sweep reversed and frequency sweep reversed waveforms is very low.
An additional requirement based on the coding for the waveform is that the waveform must be highly doppler tolerant. If the pulse modulation code is not sufficiently doppler tolerant, then at high doppler frequencies, (doppler freq.=2 v/.lambda.) very high sidelobes frequently referred to as grating lobes will be generated in the compressors. Accordingly, weak echos reflected from moving targets may be masked by these uncancelled sidelobes from strong moving targets thereby decreasing the probability of detection of weak targets by the MTI radar system.
In the prior art, only one polyphase waveform, i.e. the P-2 polyphase code disclosed in the paper "A New Class of Polyphase Pulse Compression Codes and Techniques" by B. L. Lewis and F. F. Kretschmer, Jr., IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-17, No. 3, May 1981, and U.S. patent application Ser. No. 143,399 has the characteristic of having an identical correlation function under frequency sweep reversal. However, the P-2 is derived from the step approximation to the linear frequency modulation code which is known to have inherently poor doppler tolerance at high doppler frequencies. Specifically, it is known that the P-2 code generates grating lobes which maximize for doppler shifts equivalent to range doppler coupling of p.sup.1/2 /2, i.e. when the doppler shift is on the order of one half of the fundamental frequency, where the fundamental frequency is equal to the spacing between contiguous frequencies.
Accordingly, any system using the P-2 code will be inherently limited to small doppler shifts. Moving targets with large doppler shifts may mask smaller moving target echos by their grating lobes.